1. Field of the Invention
The present invention relates to capacitor charging methods which are adaptable for measuring the insulation resistance of a capacitor and for determining whether a capacitor is acceptable.
2. Description of the Related Art
To determine whether a capacitor is acceptable, according to a known method for measuring the insulation resistance of the capacitor, a DC voltage is applied to the capacitor and the leakage current (charging current) of the capacitor is measured after the capacitor is sufficiently charged. As a matter of course, the leakage current is low in an acceptable capacitor. Then the insulation resistance can be determined from the voltage and current by Ohm's law.
An insulation-resistance measuring method of this type is specified in Japanese Industrial Standard (JIS) No. C5102. This measuring method requires a measurement time of about 60 seconds because a current needs to be measured after a capacitor has been sufficiently charged. In order for the cost of electronic units to be reduced and in order to increase their reliability, however, improvement is needed in the manufacturing capacity and the quality of electronic components such as capacitors. This requirement cannot be satisfied by the conventional measuring method, which requires such a long measurement time period for each capacitor.
In addition to the above method, which continuously applies a DC voltage, there has been known a capacitor charging method in which a DC voltage is intermittently applied (Japanese Unexamined Patent Publication No. 4-254769). This charging method is suited to a case in which a characteristic is measured with the use of a turn table which is intermittently rotated. This method can be used for continuous characteristic measurement for a number of capacitors supplied from a parts feeder. One type of insulation-resistance measuring method employing a turn table is the continuous type in which the insulation resistance of each capacitor is measured one by one when the capacitors have been charged at a plurality of charging areas. Another type of method is the batch type in which a specified number of capacitors are supplied to a turn table, the turn table is stopped, and charging and insulation-resistance measurement are performed at the same time on a plurality of capacitors. Both types of method require a long time period for charging and do not have good charging efficiency.
The inventors of this application have found after intensive research on intermittent application of a DC voltage to a capacitor that even intermittent voltage application has the same effect as continuous voltage application, under certain conditions. In other words, the same charging characteristic is obtained by intermittent voltage application as by continuous voltage application. Even if voltage application is stopped for a moment, if it is for a short period, charging still advances.
FIG. 1 and FIG. 2 show the relationship between the current and the time in cases in which a DC voltage was continuously applied to a ceramic capacitor and a DC voltage was intermittently applied to the ceramic capacitor, respectively, with coordinates being plotted along a logarithmic current scale and a logarithmic time scale and with measurement of current changes. In the continuous voltage application, as shown in FIG. 1, an almost constant, high current flowed during a short period 1, following the starting time t.sub.0. Then the current rapidly decreased during a transition period 2. And then, the current decreased with a linear charging characteristic 3 having a certain gradient. This linear charging characteristic 3 continued until one to two minutes elapsed after the charging started.
In the intermittent voltage application, as shown in FIG. 2, characteristics 1, 2, and 3 during a first voltage application were the same as those in the continuous voltage application. The voltage application was stopped at a time t.sub.a and the capacitor was discharged to ground. And then a second voltage application was performed at a time t.sub.b. The current rapidly increased with a characteristic 4 and rapidly decreased to be stable with a linear charging characteristic 5. Although the top section of the characteristic 4 cannot be clearly seen because the horizontal axis is a logarithmic time scale in FIG. 2, the section was actually formed of a level part which was the same as the characteristic 1 and a transition period which was the same as the characteristic 2. It was found that the linear charging characteristic 5 was positioned along a line which extends from the linear charging characteristic 3, which was obtained in the first voltage application. When the intermittent voltage application was repeated at a time td, the same characteristics as those shown at 4 and 5 were repeated and the current curve became stable along a line which extends from the linear charging characteristics 3 and 5. The same voltage was applied in the above continuous and intermittent voltage applications.
In each case, a current i.sub.3 was measured at a time t.sub.3 after a constant time T had elapsed from the start of both the continuous voltage application and the intermittent voltage application. The same current i.sub.3 was measured in both cases. In other words, even when a DC voltage is intermittently applied, if an OFF period (t.sub.a to t.sub.b) in the intermittent application is short (equal to or less than several hundred milliseconds, for example), the same result is obtained as when charging is performed with continuous voltage application.
According to an experiment performed by the inventors of this application, the same result was obtained as when continuous application was performed, for a capacitor having a capacitance of 0.01 .mu.F or less if the OFF time period of intermittent application was 500 ms or less.
By examining the above charging characteristics, the following fact was found. An equivalent circuit of the capacitor is formed of a capacitor C.sub.0, an internal resistor r, an insulation resistor R.sub.0, and a dielectric polarization component D, as shown in FIG. 3. It was found that the non-linear characteristics 1 and 4 shown in FIGS. 1 and 2 correspond to the charging stage of the capacitor C.sub.0 whereas the linear charging characteristics 3 and 5 correspond to the charging stage of the dielectric polarization component D.
Summarizing, it was found that the above intermittent voltage application causes the same result as continuous voltage application. In other words, the charging speed of the capacitor is the same.